Bilirubin is preferentially deposited in the brainstem and cerebellum of newborn piglets. Elevated plasma or brain bilirubin levels cause both latency and amplitude changes in brainstem auditory evoked responses (BSAER's) recorded from adult rats, newborn infants, and newborn Rhesus monkeys during naturally occurring or infusion-produced hyperbilirubinemia. We plan to study the distribution of bilirubin within the brainstem and auditory system of newborn piglets and to investigate the dose response between brainstem bilirubin concentrations and bilirubin- induced changes in synaptic neurotransmitter metabolism and BSAER. First, the anatomical location of infused bilirubin will be determined A) by histologic examination of serial brainstem sections, using the technique of Barmada and Moosy, and B) by fractioning the brain to identify bilirubin deposition in capillaries, glia, synaptic terminals, and nerve cell bodies using sucrose density gradients and 14C-labeled bilirubin. Second, the effects of elevated local bilirubin concentrations on synaptic function in the brainstem will be studied in vitro and in vivo, in synaptosomes from adult rats and prepared from the brainstems of newborn piglets. Third, the dose response effect of infused bilirubin and elevated brainstem bilirubin concentrations on BSAER will be determined in newborn piglets to establish a correlation between the anatomical distribution and subcellular localization of bilirubin in the brainstem (the region of highest bilirubin concentration in the newborn piglet), abnormalities of neurotransmitter metabolism in the newborn brainstem, and bilirubin-related pathophysiologic changes in brainstem evoked responses. The studies will provide new information concerning the pathophysiologic effects of hyperbilirubinemia on hearing and the neuronal mechanisms of bilirubin toxicity.